Photodetectors, such as a solar cell, having a thin active body, typically exhibit a conversion efficiency less than the maximum obtainable because the absorption coefficient of the body for light at a particular wavelength is too small. This effect is often compensated for by using for the back electrical contact to the body a material, such as metal, exhibiting high reflectivity so that the transmitted light is reflected back into the body. However, most contact materials do not efficiently reflect light at this interface. For example, in an hydrogenated amorphous silicon solar cell such as that disclosed by Carlson in U.S. Pat. No. 4,064,521, incorporated herein by reference, a chromium metal contact reflects only about 25 percent of the light in the wavelength region range between about 0.6 and 0.7 micrometers (.mu.m) and materials such as molybdenum and steel reflect even less light. An aluminium contact to the amorphous silicon body reflects about 73 percent of the incident light in this wavelength range but will cause degradation of the device performance if heated above a temperature of between about 100.degree. C. and 150.degree. C.
Thus, it would be desirable to have a photodetector structure in which the absorption in the semiconductor body is increased and undesirable side effects such as degradation of the device performance upon heating are reduced.